fix: {.cast(uncheckedAssign).} ineffective across yield in closure iterators (#25916)

closureiters.nim splits a stmt list at yield points, moving post-yield
code into a new state body. When that stmt list was inside a pragma
block like `{.cast(uncheckedAssign).}`, the new state's body was created
as a bare nkStmtList without the wrapper.

Fix: track the enclosing pragma block in the transform context, and wrap
newly-created state bodies in a copy of it when the split occurs inside
one. Added an explicit `nkPragmaBlock` case to
`transformClosureIteratorBody` that saves/restores `ctx.enclosingPragma`
around its body.

compiler/closureiters.nim

@@ -167,6 +167,8 @@ type
     curExcSym: PSym # Current exception
     externExcSym: PSym # Extern exception: what would getCurrentException() return outside of closure iter
 
+    enclosingPragmas: seq[PNode] # stack of pragma blocks wrapping stmtlist
+
     states: seq[State] # The resulting states. Label is int literal.
     finallyPathStack: seq[FinallyTarget] # Stack of split blocks, whiles and finallies
     stateLoopLabel: PSym # Label to break on, when jumping between states.
@@ -983,9 +985,14 @@ proc transformClosureIteratorBody(ctx: var Ctx, n: PNode, gotoOut: PNode): PNode
         for j in i + 1..<n.len:
           s.add(n[j])
 
+        var body = s
+        for pragma in ctx.enclosingPragmas:
+          body = newTreeI(nkPragmaBlock, n[i + 1].info,
+                          pragma[0].copyTree, body)
+
         n.sons.setLen(i + 1)
-        discard ctx.newState(s, true, label)
-        if ctx.transformClosureIteratorBody(s, gotoOut) != s:
+        discard ctx.newState(body, true, label)
+        if ctx.transformClosureIteratorBody(body, gotoOut) != body:
           internalError(ctx.g.config, "transformClosureIteratorBody != s")
         break
       else:
@@ -1123,6 +1130,14 @@ proc transformClosureIteratorBody(ctx: var Ctx, n: PNode, gotoOut: PNode): PNode
         finallyBody = ctx.transformClosureIteratorBody(finallyBody, finallyExit)
         dec ctx.curFinallyLevel
 
+  of nkPragmaBlock:
+    # Propagate the pragma blocks so that blocks like {.cast(uncheckedAssign).}
+    # remain effective
+    ctx.enclosingPragmas.add(n)
+    n[1] = ctx.transformClosureIteratorBody(n[1], gotoOut)
+    discard ctx.enclosingPragmas.pop()
+    result = n
+
   of nkGotoState, nkForStmt:
     internalError(ctx.g.config, "closure iter " & $n.kind)
 

tests/iter/tlowerpragmablock.nim

@@ -0,0 +1,52 @@
+discard """
+  action: "run"
+"""
+
+# Test: {.cast(uncheckedAssign).} must suppress FieldDiscriminantCheck
+# when a yield inside the pragma block causes the closure-iterator
+# transform to split the body.  The discriminant assignment lands in
+# the post-yield state and must inherit the wrapper.
+
+type
+  MyKind = enum mkOne, mkTwo
+  MyVariant = object
+    case kind: MyKind
+    of mkOne:
+      x: int
+    of mkTwo:
+      y: float
+
+iterator iterUncheckedYield(dest: var MyVariant): int {.closure.} =
+  {.cast(uncheckedAssign).}:
+    yield 1
+    dest.kind = mkTwo
+
+block:
+  var v = MyVariant(kind: mkOne, x: 42)
+  var count = 0
+  for x in iterUncheckedYield(v):
+    if count == 0:
+      doAssert x == 1
+      inc count
+      # don't break — continue to advance past the yield,
+      # which runs the discriminant assignment
+    else:
+      break
+  doAssert v.kind == mkTwo
+
+iterator iterNestedPragma(dest: var MyVariant): int {.closure.} =
+  {.cast(uncheckedAssign).}:
+    {.cast(gcsafe).}:
+      yield 1
+      dest.kind = mkTwo
+
+block:
+  var v = MyVariant(kind: mkOne, x: 42)
+  var count = 0
+  for x in iterNestedPragma(v):
+    if count == 0:
+      doAssert x == 1
+      inc count
+    else:
+      break
+  doAssert v.kind == mkTwo